This invention relates to the fabrication of extrusion dies and particularly to the fabrication of dies having an input port-to-output port contour substantially similar to the idealized flow patterns of the material to be extruded. The shape of extruded material is dependent primarily on the configuration of the dies through which the material is forced. Such dies are often referred to as profile dies. The profile dies are affixed to the exit port of an extrusion apparatus. The exit port of the extrusion apparatus is usually of a circular configuration. The dies must transform a substantially cylindrical melt of viscous material into a length of hardened material having an outer peripheral contour or profile of the desired configuration. An idealized profile die configuration would be one which would permit a gradual transition from the input port of the die to output port of the die. Both the cross sectional area and the shape change should vary uniformly as the material to be extruded progresses from the exit port of the extrusion apparatus to the final profile of the product.
The more complicated the shape of the finally extruded product, the greater is the difficulty in achieving a gradual transition between the input port of the profile die and the exit port of the profile die. The present profile dies are hand tooled and attempt to achieve a gradual transition over a short axial distance. Initially such profile dies had little or no transition region over which the material to be extruded, usually referred to as the extrudate, could be transformed from a circular melt to a configuration having the desired contour or profile. A block of material, preferably steel, having an orifice the shape and size of the extruded product was affixed to the exit end of the extrusion apparatus. This resulted in blocking part of the extrudate. This back-up or blocking of extrudate often resulted in melt fractures and burning part of the extrudate. To overcome this deficiency, the dies and the extrusion apparatus were commonly kept at very high temperatures to allow the extrudate to remain highly fluid. However, this gave rise to post-extrusion problems. The extrudate would exit from the extrusion die and be too warm to maintain the final configuration without undue distortion.
The next designs provided a cone-like transition region prior to the area in the die having the configuration of the finally extruded product. This "hog out" technique lessened the burning of extrudate and the blocking of melt prior to entering the contour area. However, such transition regions were still generally unsatisfactory for mass production applications.
A further problem was that the profile dies themselves had to be kept at a temperature sufficient to allow the material to be extruded to flow through them. However, when dies are machined from a block of steel, as is the normal method for manufacturing extrusion dies, the thermocouples which are used to check the temperature and the heater coils which are used to heat the die generally are displaced a considerable distance from the area in which the extrudate transformation is occurring. Thermocouples may be placed close to the inner surface of the die. Immersion heaters may also improve the position of application of heat. However, heating the massive steel die is one of the primary reasons for thermal lag. A considerable period of time may elapse between the sensing of a deficiency in the temperature of the die and rectifying the temperature problem. Because of the stresses and temperature of die operation, it is not generally feasible to employ materials in manufacturing profile dies which will result in a lesser thermal lag.